The effects of nitroglycerin, nitroprusside and phentolamine on coronary collateral function were tested in dogs with well-developed coronary collateral vessels. Nitroglycerin enhanced collateral flow by both a direct action on the coronary circulation and by indirect mechanisms. Although nitroprusside was more potent than nitroglycerin in reducing systemic arterial blood pressure, it was less effective in increasing coronary collateral flow. Phentolamine failed to increase collateral flow.